1. Field of the Invention
The present invention relates to a separation membrane which is superior in gas permeability, heat resistance, durability, and membrane-forming property.
2. Description of the Related Art
Polymer membranes for separation and concentration of substances have been studied comprehensively for the purpose of energy saving, effective utilization of resources, and so forth.
High polymer materials are generally known to allow gases to pass through. However, the permeability thereof is usually low, and varies greatly depending on the permeating substances. In particular, the materials showing oxygen permeation coefficient (PO.sub.2) of higher than 1.times.10.sup.-9 cm.sup.3 (STP).cm/cm.sup.2.sec.cmHg are limited to only few substances such as polydimethylsiloxane, poly(4-methylpentene-1), polyphenylene oxide, poly[1-(trimethylsilyl)-1-propyne], and the like.
Of the above substances, poly[1-(trimethylsilyl)-1-propyne] is known to have the highest permeation coefficient among organic high polymers. This substance, however, involves disadvantages of remarkable aging deterioration, low heat stability, low durability, low permeation selectivity, and so forth.
On the other hand, the above-mentioned polydimethylsiloxane, which has low glass transition temperature and high gas permeability, involves disadvantages such that the substance per se has low mechanical strength and is lacking in membrane-forming property.
After comprehensive studies, it was found that a membrane comprising a certain N-substituted maleimide polymer has high gas permeability, high heat resistance, high durability, and superior membrane-forming property, and thereby the present invention has been completed.
Generally, N-substituted maleimide polymers are known to have high glass transition temperature and high heat decomposition temperature and to give a material superior in thermal properties. The investigations have been made mostly on N-aromatic-substituted maleimides. These maleimide polymers, however, are brittle and inferior in moldability.
It was found by the inventors of the present invention that the introduction of a flexible group as the N-substituent gives a polymer which is improved in brittleness and which gives a membrane having satisfactory mechanical strength and yet having high gas permeability.